Electron paramagnetic resonance (EPR) imaging has been recently recognized as an important tool for non-invasive imaging of free radicals and REDOX (reduction/oxidization) metabolism. In principle, electron paramagnetic resonance may be observed at frequencies of a few MHz in magnetic fields of a few Gauss, up to the microwave region in a magnetic field of a few thousand Gauss. Traditionally, the latter frequency region is often chosen because the signal-to-noise ratio is usually much improved with the use of relatively high magnetic fields, which implies a relatively high Lamour frequency. However, the use of microwave radiation (e.g., in the 1 GHz to 60 GHz region) is currently known to require a special resonance cavity that is not suitable for non-invasive imaging of a large size living animal, such as a human. For example, the motion of an animal in a resonance cavity, such as motion due to respiration or a beating heart, may cause changes in the resonance frequency of the cavity. In addition, generally the skin depth decreases as the frequency of the electromagnetic radiation increases, which may preclude imaging within regions of interest in a human for EPR imaging systems operating in the 1 GHz to 60 GHz region. Furthermore, high magnetic fields often pose a safety hazard.
The high frequency microwave radiation of prior art EPR imaging systems appears to pose a physical and biological hindrance to the advance of EPR imaging technology for large size living animals. Recently, there has been work in developing an EPR imaging system that employs low magnetic fields (e.g., on the order of 10 mT) at radio frequencies (e.g., about 300 MHz) where the RF (radio frequency) energy may penetrate into biological objects. However, EPR techniques developed for clinical applications to date still rely on conventional RF detection schemes which are believed to degrade signal-to-noise ratio at low magnetic fields.